The separation of trivalent actinides like americium (Am) or curium (Cu) from trivalent lanthanides has been a problem for a long time. The difficulties derive from the fact that the elements to be separated are very similar chemically and the trivalent ions of lanthanides and actinides also have similar physical properties. Especially the very similar ionic radii contribute to similar properties of both element groups. Thus a large number of investigations have already been carried out to attempt to separate with the highest possible selectivity and high efficiency the trivalent ions of actinides and lanthanides.
It is indeed known that extraction agents with soft donor groups, which contain nitrogen or sulfur as complex-forming structural components have a certain selectivity in liquid-liquid extraction with respect to trivalent actinides but can carry out the separation hitherto only at relatively high pH values at which the trivalent actinides tend toward precipitate formation.
In accordance with the publication K. L. Nash, Solvent Extraction, Ion Exchange. 11(4), 729-768 (1993), the known Talspeak Process is a selective extraction of lanthanides with the aid of complexing agents which hold the trivalent actinides in solution. However, this separation process also requires relatively high pH values of 3 to 4 and requires the addition of further salts.
A process available from the Applicant for the extraction of trivalent actinides from aqueous solutions which contain trivalent actinides and trivalent lanthanides which enable a separation at high acid concentrations of 0.01 to 2 mol/l HNO3 (EP 1 019 552 B1) in accordance with this process, such an acid is aqueous solution which contains a mixture of trivalent lanthanides and actinides, is extracted by means of an organic solvent which contains as the extraction agent, a bis(aryl)dithiophosphinic acid to which a synergist is added. With this method, because of the low pH value or high acid concentration, a precipitation of the trivalent actinides is limited and especially good separation results are obtainable. To evaluate the separation effect, the distribution coefficient D and the separation factor SF are considered.DAN(III)=[AN(III)org]/[An(III)w]  (1)
In formula 1:                DAN(III)=The distribution Coefficient for Trivalent Actinide (dimensionless)        [AN(III)org]=Concentration of the Trivalent Actinide in the Organic Phase (mol/l)        [An(III)w]/=Concentration of the trivalant actinide in the aqueous phase of mol/liter).DLN(III)=[Ln(III)org]/[Ln(III)w}]  (2)        
In formula 2:                DLN(III)=Distribution Coefficient for a trivalent lanthanide (dimensionless)        [Ln(III)org]=Concentration of the trivalent lanthanide in the organic phase (mol/liter)        [Ln(III)w}]=The concentration of the trivalent lanthanide in the aqueous phase (mol/liter)SF=DAN(III)/DLN(III)  (3)        SF=Separation Factor (Dimensionless)        
With a separation factor of SF=1, no separation is possible.
In accordance with that process using bis(aryl)dithiophosphine acid in strongly acid medium, the separation factor SF between trivalent actinides and trivalent lanthanides, depending upon the synergist, lies between 20 and 2000.
There exist various methods for the separation of the trivalent actinides (An)(III)Am and Cm from highly active liquid wastes, the Raffinate of the Purex Process. Madic, C. In 6th Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation, Madrid, Spain, 11-13 Dec. 2000, EUR., 19783, 2001, S. 53 to 64.
In the first step the An III are coextracted together with the lanthanides Ln(3) and the greater part of the fission products (for example Mo, Zr, Cs, Fe, etc. are separated out. The TRUEX, TRPO and the DIAMEX-processes belong to those known processes in all of the these processes (An/III)/Ln(III) such fractions are obtained that require further separation, hitherto with considerable problems. Up to know there has been no extraction agent which can selectively separate An(III) from Ln(III) of strongly acidic solutions. If one thinks of the P & T Nuclear Fuel Circulation in which the long lived actinides are first separated off (Partitioning, P) and then eliminated by transmution T, a An(III)/Ln(III)—separation is impermissible on the following grounds.
About a third of the fission products are lanthanides.
The Ln(III) have the same oxidation state (III) as the An(III) and the chemical characteristics are very similar to those of the AnIII.
Several lanthanides have very large neutron capture cross sections and act as neutron poisons with respect to the transmutation.
In the DIAMEX-Process, the An(III) and Ln(III) are extracted together from the 3 to 4 mol/L HNO3 containing Purex-Raffinate with the aid of melanic acid diamide. After the back extraction with dilute HNO3, one obtains a product which is comprised primarily of the An(III) and Ln(III) as well as an HNO3 concentration of about 0.5 mol/liter (Madic, C.; Hudson, M. J.; Liljenzin, J. O.; Glatz, J. P.; Nannicini, R.; Facchini, A.; Kolarik, Z.; Odoj, R. New partitioning techniques for minor actinides, European report, EUR19149, 2000).
The present Applicant has also developed a method for actinide-lanthanide separation from nitric acid solutions (0.5 to 1.0 mol/liter HNO3) this process is known under the name ALINA for (Actinide (III)-Lanthanide(III) Intergroup separation in Acidic Medium). (Modolo, G.; Odoj, R.; Baron, P. The ALINA Process for An(III)/Ln(III) Group Separation from strong Acidic Medium, Proceedings of Global 99, International Conference on Future Nuclear Systems, Jackson Hole, Wyo., USA, Aug. 29-Sep. 3, 1999; American Nuclear Society, Inc.).
As the extraction agent a mixture of aromatic dithiophosphinic acid and trioctylphosphinic oxide (TOPO) in tertiary butyl benzene is used. Details for the synthesis of the new extraction agent, its radiolitic stability and its suitability for AN(III)/Ln(III) separation are disclosed in (Modolo, G.; Odoj, R. Synergistic selective extraction of actinides (III) over lanthanides form nitric acid using new aromatic diorganyldithiophosphinic acids and neutral organophosphorus compounds. Solvent Extr. Ion Exch. 1999m 17 (1), S. (33-53).
The separation of Americium and Curium is however still more difficult than Actinide (III)-/Lanthanide (III) separation and has even greater requirements or process chemistry for the following reasons:
In the trivalent state, Americium and Curium have very similar chemical properties based upon their practically identical ionic radii (0.99 or 0.986 Angstrom). Americium is found in its higher oxidation states (IV, V, VI) as a very strong oxidizing agent and the nonhydrometallurgical (liquid-liquid extraction) for example the PUREX process) do not always allow separation of the heterovalent actinide because of the instability of all oxidation states up to the trivalent oxidation state. For that reason the structure does not describe any hydrometallurgical process which can separate americium and curium form one another in their natural oxidation states.